How to unwrap a cylinder to rectangles instead of a circle?
For this you need to create so called seams.
Seams define where an objects uv layout is interrupted.

Imagine like you cut that object at those seam lines to then lay it flat on the ground.

In a cylinder you need 1 vertical seam in your case. This will allow it to be a rectangle since it can be layed out flat.

To do so press alt+(optional shift)+rigth click (or left click). This will select the whole edge loop. Now that you have done that you can press ctrl+e and go to "mark seam". It should be highlighted in red now. If the cylinder has a face on top and bottom you need the whole circle to be marked as seam too.

Now press U and select unwrap

Alternatively but not recommended you can press automatic unwrap when pressing u. This will automatically set seams where the angle of faces is over a threshold. I highly advise to not use this but do it manually as this will, not just can but will create odd at best and horrible at worst uv maps. It does not know structure and logic but only knows angles. It's OK for dead simple things like cubes but there it ends.